A diabetes mellitus epidemic is unfolding across the globe with the World Health Organization (WHO) reporting a worldwide prevalence of 177 million patients with diabetes. It is estimated that the incidence of all forms of diabetes totals approximately 2.8% of the world population. The number of newly diagnosed diabetic patients is increasing by 4-5% per year. The total number of people with diabetes worldwide is projected to rise to 366 million (4.4% prevalence) in 2030. Type 2 diabetes accounts for approximately 95% of all diabetes cases. Long-term complications of Type 2 diabetes include atherosclerosis, heart disease, stroke, end-stage renal disease, retinopathy leading to blindness, nerve damage, sexual dysfunction, frequent infections, and difficult-to-treat foot ulcers, sometimes resulting in lower limb amputation. Diabetics are twice as likely to develop cardiovascular disease or have a stroke, 2 to 6 times more likely to have transient ischemic attacks, and 15 to 40 times more likely to require lower-limb amputation compared with the general population. The total estimated cost of diabetes in 2007 in the United States was $174 billion, including $116 billion in medical expenditures. The largest components of medical expenditures attributed to diabetes are hospital inpatient care (50% of total cost), diabetes medication and supplies (12%), retail prescriptions to treat complications of diabetes (11%), and physician office visits (9%). This may be related to the lack of durable efficacy of current drug therapies for Type 2 diabetes (>50% Type 2 patients are not reaching the targeted blood glucose control with current oral medications after 5 years of treatment). There is a general consensus that a considerable need exists for improved awareness, diagnosis and new, more effective, drug therapies for diabetes.
GLP-1 is secreted from specific cells in the colon following a meal and is a key regulator of glucose homeostasis, linking the gut, brain and pancreas. GLP-1 potentiates insulin secretion, reduces glucagon secretion and preserves β-cell function whilst also improving satiety. Levels of post-prandial GLP-1 are reduced in Type 2 diabetics and dramatically elevated following gastric by-pass surgery, contributing to the amelioration of Type 2 diabetes in these patients. Approaches that prolong the half-life of GLP-1 (JANUVIA (Merck), GALVUS (Novartis)) or activate the GLP-1 receptor (BYETTA (Amylin)) have been recently approved for use in Type 2 diabetes.
Hyperinsulinemia in patients with Type 2 diabetes mellitus results from peripheral insulin resistance, coupled with inadequate pancreatic insulin secretion and elevated glucagon levels. There is a strong correlation between obesity and peripheral insulin resistance and hyperinsulinemia. Accumulation of free fatty acids in insulin responsive tissues other than fat (i.e. muscle and liver) results in tissue insulin resistance. Additionally, free fatty acids have a direct effect on the pancreas and in the colon and further stimulate glucose-dependent insulin secretion and GLP-1 release with acute exposure whereas chronic exposure of free fatty acids impairs insulin secretion and becomes toxic to the β-cell. In the liver, hyperinsulinemia per se has been linked to exacerbating insulin resistance by increasing liver fatty acid accumulation and hepatic glucose output creating a vicious cycle of disease progression. Current therapeutic strategies only partially address the complex pathology of free fatty acids in the exacerbation of diabetes. Agents that target both liver and pancreas function, directly or indirectly via GLP-1 release, either individually or in combination with current treatment, could significantly improve blood glucose control while maintaining β-cell function. Agents that potentiate GLP-1 release also have the ability to reduce weight, providing additional benefits.
GPR120 is a seven transmembrane g-protein coupled receptor (GPCR) that is predominantly expressed in the intestine and adipose. GPR120 functions as a receptor for long chain free fatty acids (FFAs). Acute FFA stimulation of GPR120 in GLP-1 expressing cell-lines amplifies GLP-1 release. Administration of α-linolenic acid into the colon of mice increases GLP-1 and potentiates insulin release following a glucose challenge. In contrast to agonists of GPR40, the existing literature suggests that a GPR120 agonist would potentiate insulin secretion and reduce glucagon indirectly via GLP-1 release. GPR120 is also expressed in adipose, with expression induced during differentiation Inhibition of GPR120 expression in 3T3-L1 adipocytes has been shown to reduce adipocyte differentiation. The role of the receptor in the adipose or in the taste cells of the tongue, where it has also been found, remains unclear.
GPR120 is a Gq coupled GPCR that acts a receptor for long chain fatty acids. It belongs to a family of lipid binding GPCRs that include GPR 40, 41, 43. Functionally, GPR120s closest homolog is GPR40. The cloned rat and mouse GPR120 receptors have been cloned and have >85% homology with the human receptor. GPR120 signals through Gq to elevate intracellular Ca2+ levels as well as activate MAP kinase signal transduction cascades. GPR120's activation of calcium flux and PKC activation is most likely how FFAs contribute to the release GLP-1 in the L-cell.
Although relatively little is known about GPR120 due to a lack of potent, selective pharmacological tools or a documented metabolic phenotype of GPR120 knockout mice, the potential to elevate GLP-1 from a small-molecule perspective is attractive as a novel approach to an unmet medical need in the treatment of Type 2 diabetes mellitus and related disorders. The beneficial effects of elevating GLP-1 are already well validated in the clinic and in addition to improved glucose homeostasis, offer the potential of weight loss. Thus GPR120 agonists may be complementary to existing diabetes therapies that affect liver insulin sensitivity and those that preserve β-cell function.